HI
I think fresh flowers which I get at Trader Joes for $4 is probably a nice touch - that and a small chalkboard on which I write, "Welcome John!" are the first things they see when they open the door.
I leave the light on so that it is never dark when they arrive. The AC is on if needed.
I provide a lot of non-perishable food items which they can pick from e.g. microwave popcorn, ramen noodles (very popular!), cereals, hot chocolate mix, jams, peanut butter, coffee and tea. Each set of guests gets a package of English muffins (cheap and disposable if they don't eat them) along with a fresh half stick of butter, and half and half for coffee (I have a Keurig coffeemaker, don't yell at me! I know it's not ecological...) I have oil and vinegar and a range of basic spices, and all of those things last through multiple guests.
I have a small drawer with some basic over the counter medicines and sunscreen - Aspirin, headache tablets, stomach tablets, Bandaids, allergy tablets, cortisone cream, etc. When you open the cupboards they are full of "goodies". Also the kitchen is completely equipped with decent pots, knives etc.
For kids I invested in a couple of inflatable kid-sized beds, a bucket of blocks and a couple of puzzles, and some new boxes of crayons (I remember what a thrill it was to get a new box of crayons!), a child booster seat for the table, and a few books which are kid-oriented.
I have a MBTA (transit) pass which I lend them, but they often forget to return it (but some then return it via mail!)
People often comment that the place is so well provided, they couldn't think of anything which they needed which wasn't there.